Some years ago, John Belrose VE2CV published an article in QST about
off-center fed antennas, based on a combination of modeling and
building. While they look attractive on a simulation program (where the
feedline wasn't also modeled), he found that in practice it was very
hard to keep the feedline common mode current down to a low value. The
consequence of common mode current is that the feedline becomes part of
the antenna, and that makes it very difficult to duplicate or to realize
the idealized performance you might expect if you hadn't taken feedline
radiation into account. Multiple current baluns should be able to reduce
the common mode current to a reasonable value, but they'd have to be
used unless you want a lot of feedline radiation and performance that's
difficult to predict.

Roy Lewallen, W7EL

Buck wrote:
On Wed, 20 Apr 2005 23:21:29 -0500, (Richard
Harrison) wrote:


Cecil, W5DXP wrote:
"There would be three current maximum points on a 3/2WL wire."

Yes, and I believe they occur 1/4-weave back from the open-circuit ends
of the antenna and at the very center of the antenna.

Best regards, Richard Harrison, KB5WZI


Thanks, That's what I was looking for. so I can use the antenna with
1/4 - 3/4 or center loaded.

Would the off-center setting create another resonant frequency for the
antenna?

Assuming the antenna were cut for 1/2 wave at 7 MHz, it would operate
on 21 MHz, but if off-center fed, would it also resonate on 20 MHz
with the 1/4 wave element?

Thanks.

A bit more on the baluns Roy mentioned...

If the feedpoint is a high impedance (an even number of half-waves back
from both ends of the antenna), it's difficult to get enough choke
impedance to do much good if you put a single balun right at the
feedpoint. It may end up being overkill, but if you don't have a good
way to measure your line for antenna currents, you'll do well to put
chokes about a quarter wave apart at the highest operating frequency.
You want to avoid having a length of line between two chokes being a
resonant half-wave long without anything in the middle to break up the
resonance. So for operation on multiple bands covering a wide range,
you should think about putting in three or more chokes.

Of course, if you put it up without any chokes and it does what you
want, that's fine...just don't expect it will match the model of an
isolated wire.

Cheers,
Tom

On Thu, 21 Apr 2005 10:56:46 -0700, Roy Lewallen
wrote:

Some years ago, John Belrose VE2CV published an article in QST about
off-center fed antennas, based on a combination of modeling and
building. While they look attractive on a simulation program (where the
feedline wasn't also modeled), he found that in practice it was very
hard to keep the feedline common mode current down to a low value. The
consequence of common mode current is that the feedline becomes part of
the antenna, and that makes it very difficult to duplicate or to realize
the idealized performance you might expect if you hadn't taken feedline
radiation into account. Multiple current baluns should be able to reduce
the common mode current to a reasonable value, but they'd have to be
used unless you want a lot of feedline radiation and performance that's
difficult to predict.

Roy Lewallen, W7EL

A few years back I put up an OCFD. Feed it with open line. Seemed to
work pretty well and behaved itself running less than about a
100-watts. However, when I increased my power to above 500-watts I was
able to work every electrical and electronic appliance in the house!
The common mode on the transmission can be a killer!

Danny, K6MHE

In article ,
says...


On Thu, 21 Apr 2005 10:56:46 -0700, Roy Lewallen
wrote:

Some years ago, John Belrose VE2CV published an article in QST about
off-center fed antennas, based on a combination of modeling and
building. While they look attractive on a simulation program (where the
feedline wasn't also modeled), he found that in practice it was very
hard to keep the feedline common mode current down to a low value. The
consequence of common mode current is that the feedline becomes part of
the antenna, and that makes it very difficult to duplicate or to realize
the idealized performance you might expect if you hadn't taken feedline
radiation into account. Multiple current baluns should be able to reduce
the common mode current to a reasonable value, but they'd have to be
used unless you want a lot of feedline radiation and performance that's
difficult to predict.

Roy Lewallen, W7EL

A few years back I put up an OCFD. Feed it with open line. Seemed to
work pretty well and behaved itself running less than about a
100-watts. However, when I increased my power to above 500-watts I was
able to work every electrical and electronic appliance in the house!
The common mode on the transmission can be a killer!

Danny, K6MHE


If I was going to run open line, I would not go with
an off-center feed. I don't think it buys you much.
On the other hand, off-center feed with coaxial feedline
can buy bandwidth over center feed. Our 80/75 dipole
is off-center fed with a 4:1 voltage balun followed
by a bead balun on the feedline. The SWR is quite low
from 3.5 to at least 3.8 MHz. Solid-state finals and
Alpha 87A finals like it very much. No trouble with
getting into appliances at 1500 watts.

73,
Bob
K3PH

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